Recent advances in solid state conversion technology has led to the proliferation of variable frequency induction motor drives (VFIMD's) that are used in various applications such as air conditioning, blowers, fans, pumps for waste water treatment plants, textile mills, rolling mills etc.
As a practical technique, direct torque control (DTC) strategy is implemented in induction motor drives (DTCIMDs), serving various applications. These drives utilize voltage source inverters which are fed from conventional six-pulse diode bridge rectifiers. The most important drawback of these rectifiers is their poor power quality injection of harmonic currents into ac mains. The circulation of current harmonics into the source impedance yields in harmonically polluted voltages at the point of common coupling (PCC) and consequently resulting in undesired supply voltage conditions for the nearby costumers. The value of current harmonic components which are injected into the grid by nonlinear loads should be controlled within the standard limits.
The most prominent standards in this field are IEEE standard 519 and the International Electro-technical Commission (IEC) standard. For DTCIMDs one effective solution to eliminate harmonics is the use of multipulse AC-DC converters. According to the recent investigations, these converters are based on either phase multiplication, phase shifting, pulse doubling or a combined solution (have been reported in U.S. Pat. No. 7,274,280). Application of multi-pulse technique (up to 18-pulse) in AC-DC converters are reported in U.S. Pat. No. 7,375,996 where line current THD of more than 5% is experienced under different load conditions.
The polygon-connected autotransformer based 30-pulse (U.S. Pat. No. 7,719,858) was designed for AC-DC power converter. The DC link voltage in this topology is higher than that of a 6-pulse diode bridge rectifier, thus making the scheme non-applicable for retrofit applications. However, the polygon-connected autotransformer based configurations, make use of three single-phase transformers while the T-connected autotransformer topology employs only two single-phase transformers, resulting in reduced space, volume, weight, and, finally, cost of the drive (U.S. Pat. No. 7,375,996).
In this invention, design of a T-connected autotransformer based 20-pulse AC-DC converter is proposed. In the proposed structure, two five-leg diode-bridge rectifiers are paralleled via a Zero Sequence Blocking Transformer (ZSBT) resulting in a 20-pulse output voltage. In order to double the number of pulses up to 40, a Tapped Inter-Phase Transformer (IPT) with two additional diodes are added at the rectifier outputs. The proposed converters are modeled and simulated using MATLAB to study its behavior and specifically to analyze the power quality indices at ac mains. Finally, a small-rating laboratory prototype of the proposed 40-pulse converter is constructed in order to verify the simulation results and examine the effectiveness of the proposed topology.